Receiver selectivity control circuit



to a common carrier frequency and hence imas the construction and function of the pre- -70 35 frequency with the result that a broadened overtribution line or a capacitive antenna device of 35 I v-10 tiv 1y control device for the cascaded tunable quency input circuit 2. The input circuit 2 com- 40 Yet another object of the invention be this casea tube of the 6A7, or 2A7 type, may be Patented Mar. 26,1940

UNITED STATES PATENT ()FFIC'E RECEIVER SELECTIVITY CONTRQL CIRCUIT Garrard Mountjoy, Bayside, N. 1. assignor to Radio Corporation of America, a corporation of Delaware Application Marni 12, 1931, Serial No. 130,458 7 Claims. (o1. lit- 14) My present invention relates to selectivity conwidth control device is employed to regulatethe trol circuits for radio'receivers, and more parresonance curve characteristic of the IF netticularly to a novel method of, and'means for, i work, and the two control devices being mecontrolling the selectivity of the preselector circhanically coupled in such a manner that the 5 cults of a receiver, I radio frequency circuits of the receiver are 1 5 Band width control devices, or fidelity adjustbroadened 'at a predetermined angle of adjustment means, are well known in the art of radio ment of the control device for the IF network. reception; such devices are generally employed to The novel features which I believe to be charincrease the radio frequency selectivity of a reacteris'tic of my invention are set forth in parce'iver, and decrease the fidelity of signal reproticularity in the appended claims; the invention, duction, when distant, or weak stations are re itself, however, as to both its organization and ceived. Conversely, the band width control de-- method of operation will best be understood by vice is generally'used .to decrease the selectivity, reference to the following description taken in and thus increase 1e fidelity of'signal reproducconnection with the drawing in which I have 15 ticn, when strong, or local, stations are received. indicated diagrammatically a circuit organiza- 15 The fidelity control device has generally been 10- tion whereby my invention may be carried'into cated in a signal selection circuit, and adjustment effect. of the control device results in a variation of the i In the drawing: resonance curve characteristic of the selection Fig, l is a schematic circuit diagram of a supercircuit. Obviously, a fidelity control device heterodyne receiver embodying the invention, 20

should have a minimum number of elements, and I and function in a reliable manner to regulate the Fig 2 graphically represents the electrical opselectivity of the receiver. eration of the selectivity control device for the Accordingly, it may be stated that itis' one of pie-selector circuits. i

the primary objects of my present invention to 3 Referring now to the circuit diagram shown 25 provide a selectivity control device which is conin Fig. 1, it is first pointedout that the receiver structed ofa minimum number of elements and shown therein is of the superheterodyne type. acts during one portion of its adjustment to It is to be clearly understood that the receiver maintain the receiver preselector circuits tuned can be of the tuned radio frequency type insofar parts desirable selectivity to the receiver; and selector selectivity control device are concerned. during another portion ofitsadjustment it func- The receiver comprises the usual signal collector tions oppositely to detune at least two cascaded A, and the. latter may be a grounded antenna pie-selector circuits from the common carrier circuit, a loop antenna, a radio frequency disall resonance curve characteristic is obtained for l the type usually employed on mobile vehicles, the pre-selector circuits. such as automobiles. The radio frequency am- Another important object of the inventionmay plifier l, which may have one or more tubes, is be stated to reside in the provision of a selecshown as being provided with a tunable radio fre- .radiofrequency circuits of a receiver, wherein prises an inductance coil t and'avariabletuning the control device comprises an inductive recondenser d. The output energy of, the radio actance electrically associated with the inducfrequency amplifier l is transmitted to the tuntive reactance of each one of the cascaded radio able input circuit 5 ofthe converter network 6.

frequency circuits. and a single switch device be- The circuit 5 comprises the inductance coil 1 ingemployed for-rendering either one of the and the variable tuning condenser 8.

. inductive reactanees effectiveas a short-circuit- To preserve simplicity of disclosure :thecon- 1 ing inductance with respect to its associated radio verter network has been shown as of the comfrequency circuit. bined local oscillator-first detector type, and in stated to reside in the provision of a selectivity employed in the circuit. The numeral 9 denotes control device which is essentially utilized to l the tunable local oscillator tank circuitof the control the overall resonance curve characteristic converter, and the reference numeral l0 denotes of. thetunable radio frequency circuits of a superthe usual mechanical uni-control device which n 5 heterodyne receiver, while an auxiliary band functions simultaneously to vary the rotors of 56 the variable condensers used in the signal circuits and the local oscillator tank circuit. Those skilled in the art are fully aware of the fact that the frequency range of the tank circuit 9 will difier from the common frequency range of signal circuits 2 and 5 by a frequency value which is equal to the operating IF. If the receiver is of the broadcast type, which means that the signal frequency range is from 500 to 1500 kc., then the IF may have a value chosen from a range of 175 to 465 kc. The resonant output circuit ll of the converter Bis fixedly tuned to the operating IF, and it is coupled to a following I resonant circuit l2 whichis also tuned to the IF. One or more stages of IF amplification may follow the IF network |l-l2, and the usual second detector and audio amplifier circuit, terminated by a reproducer, will be included in the receiver.

Itwill be understood by those skilled in the art that the receiving circuit shown in Fig. 1, and described up to this point, is of a convention- 'al type. Many variations in construction of the 'receiver are possible. For example, the radio "frequency amplifier may comprise several tunable amplifiers in cascade, and the converter network may employ separate first detector and local oscillator tubes. The tuning mechanism ill denoted "as Tuner, in Fig. 1, is manually adjusted so that the desired stationsmay be received. The degree of coupling between IF circuits H and 12 may be manually adjusted in any. desired manner, and the numeral l3 designates in con- 'ventional manner a band width control device which may be provided in the IF network to adjust the band width. It will be obvious to those skilled in the art that the adjustment of the degree of coupling between, circuits I! and I2 will result in varying the resonance curve characteristic of the IF network from a single-peak curve to a wide, double-peak curve.

The single-peak curve, secured by coupling between circuits H and I2 which is critical or less than criticalcoupling, is employed when receiv ing weak, or distant, stations. By increasing the degree of coupling between circuits H and I2" to -m'ore than critical coupling the resonance curve characteristic is broadened and may even have a double peak. As stated heretofore, band width control devices are well known and are usually provided to control the fidel ity of signal reproduction of a receiver. At wide-band adjustment of device l3 practically all the modulation side bands are transmitted device I3 is adjusted to its high selectivity adju'stme'nt, then many of the higher audio frequencies are prevented from reaching the second detector. 1

It is often desirable, in order to insure efiicient and satisfactory signal reproduction under different conditions of signal reception, to provide con- 7 "trol of the selectivity of the radio frequency preselector circuits of the receiver. Consideringthe pre-selector circuits 2 and 5 in Fig. 1, there is shown a simple and. highly effective arrange ment for controlling the receiver selectivity. The coil It is physically positionedadjacent coil 3 'so as to secure reactive coupling between the two coils. A similar coil i5 is positioned adjacent coil 1 so that the latter two coils are reactively coupled. A manually adjustable metallic contactor it is provided for slidable contact with ..a pair of metallic contacts H and 98. The con- "tactor lB'is grounded, and one of the contacts to the second detector. On the contrary when i1 is connected by lead [9 to one terminal of coil Id, whereas the opposite terminal of the coil is grounded. The contact I8 is connected by lead 20 to one terminal of coil l5 and the opposite terminal of the coil is at ground potential. It will be noted that the contacts I! and i8 are arcuate in configuration, and that the contactor it will be in contact with either of the contacts H and I8.

By means of this arrangement the coil M functions as a short-circuited Winding with respect to coil 3, as long as contactor I6 is in contact with clement l1. When'the contactor i6 is adjusted into contact with element it then the shortcircuiting effect of coil I4 is eliminated, and coil f comesunder the influence of the short-circuitingcoil 5. The effective inductances of coils 3 and 7 are chosen so that circuits and 5 track when contactor I6 is adjusted to the maximum selectivity position; that is to say when the contactor I6 is in contact with element ll. In Fig. 2 the curve designated as Selectivity denotes the overall resonance curve characteristic of pre-selector circuits 2 and 5 when contactor it is adjusted to contact element ll. It will be observed that in this position of the contactor the efiect of the short-circuiting coil it is to decrease the inductance in circuit 2, with respect to its normal non-short circuited state,

and thus increase the resonant frequency of the circuit at the particular setting of the variable condenser 4.

When the contactor it is adjusted to contact element It, the short-circuiting effect of coil M is removed, and the resonance curve characteristic of circuit 2 is denoted. by the curve a the pre-selector band characteristic has been spread by adjusting contactor l6 into contact" with element l8.

The band width control device l3 and the contactor 16, if desired, may be mechanically coupled in the manner, designated by the dotted line 2! in order to provide predetermined relation between the adjustment of device 13 and contactor it. For example, the adjustment may be such that the fidelity adjustment of contactor i6 is not had until the device 13 has been ad.- justed through a predetermined portion of its control range. It is desirable for smooth timing characteristics (no whistles, etc.), to have good I pre-selector selectivity. Only when. the IF amplifier, which is usually much more selective than the radio frequency circuit, is very broad does the radio selectivity contribute an appreciable percent of the total high frequency attenuation. When the IF amplifier is broad, or control 2| well advanced, then radio selectivity should be sacrificed to add the last contribution to fidelity. Hence, the switch-over of lficontacting l8 should occur at an advanced position. On the other hand, the contactor 16 may be employed independently of device l3. It will be noted that, when the ccntactor I6 is in contact with element Simultaneously the shortl'l' the pre-selector circuits 2 and 5 are brought into resonance with each other, and hence the gain of the stages including these circuits is materially increased. It is to be understood that the short-circuited coil can be coupled to the second radio frequency circuit, and the opencircuited coil coupled to the first radio frequency circuit, in the selective position of contactor l6. While I have indicated and described one system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is: i

In combination with a pair of cascaded resonant circuits, each resonant circuit including a coil and tuning condenser arranged in shunt relation, an auxiliary coil disposed in coupled relation to the coil of one of 'said circuits, at second auxiliary coil disposed in coupled relation to the coil of said second of said resonant circuits, means for selectively rendering either of said auxiliary coils eifective as a short-circuiting winding with respect to its associated coil one of said auxiliary coils having a magnitude such that said pair of circuits are tuned to a common frequency when said one coil acts as a short-circuiting winding, and the other auxiliary coil having a magnitude such that said circuits are each detuned from the frequency when it acts as short-circuiting winding.

2. In combination with a pair of cascaded resonant circuits, said circuits being tuned sub-' stantially to a common carrier frequency, each resonant circuit including a coil and tuning condenser arranged in shunt relation, an auxiliary coil disposed'in coupled relation to the coil of one of said circuits, a second auxiliary coil dis posed in coupled relation to the coil of said second of said resonant circuits, means for selectively rendering either of said auxiliary coils effective as a short-circuiting Winding with respect to its associated coil, each of said auxiliary coils having a terminal connected to a contact, the opposite terminals of the auxiliary coils being grounded, and said means including a rounded contactor adapted to be connected to either of said contacts.

3. In a radio receiver which includes at least two tunable signal circuits arranged in cascade, each of said signal circuits including a coil and a tuning condenser, the second of said signal circuits being tuned to a desired signal frequency, an auxiliary circuit reactively coupled to said first signal circuit to a sufficient extent substantially to change the frequency thereof and including an adjustable element for selectively opening or closing the auxiliary circuit,

and said first signal circuit being effectively tuned to said desired signal frequency when said adjustable element is in position solely to close the auxiliary circuit.

4. In a radio receiver which includes at least two tunable signal circuits arranged in cascade, each of said signal circuits including a coil and a, tuning condenser, the'second of said signal circuits being tuned to a desired signal frequency, an auxiliary circuit reactively coupled to said first signal circuit to a sufiicient extent substantially to change the frequency thereof and in cluding an adjustable element for selectively opening or closing the auxiliary circuit, said,

first signal circuit being effectively tuned to said desired signal frequency when said adjustable element is in position solely to close the auxiliarycircuit, and a second auxiliary circuit reactively coupled to said second signal circuit,

opening or closing the auxiliary'circuit, said first.

signal circuit being effectively tuned to said desired signal frequency when said adjustable element is in position solely to close the auxiliary circuit, a second auxiliary circuit reactively coupled'to saidsecond signal circuit, and including said adjustable element to control the eifect of said second auxiliary circuit on the resonant frequency of said second signal circuit,

and the frequencies of said'two signal'circuits being positively detuned from said desired signal frequency by equal frequency values, when the adjustable element is adjusted to open the first auxiliary circuit and close the second auxiliary circuit.

6. In a radio receiver having at least three cascaded resonant circuits; means for varying the selectivity of the third of the circuits, means electrically coupled with the first two circuits for varying the selectivity thereof as a unit, and additional means for adjusting said two means,

in the same sense but in successive relation such that said second selectivity varying means is adjustable over'solely a portion of the first varying means range.

7. In a radio receiver having at least three cascaded resonant circuits, -means for varying the selectivity of the third of the circuits, means electrically coupled with the first two circuits for varying the selectivity thereof as a unit, and additional means for adjusting said twomeans in the same sense but in successive relation such that said second selectivity varying means is adjustable over'solely a portion of the first varying means range, the second varying means comprising a device for controlling the tuning of said first two circuits carrier frequency.

GARRARD MOUNTJQY.

with respect to a common 

